Electrochemical devices or systems of the type referred to herein include one or more of the metal-halogen battery systems, such as a zinc-chloride battery system. These metal-halogen battery systems generally are comprised of three basic components, namely an electrode stack section, an electrolyte circulation subsystem, and a store subsystem. The electrode stack section typically includes a plurality of cells connected together electrically in various series and parallel combinations to achieve a desired operating voltage and current at the battery terminals over a charge/discharge battery cycle. Each cell is comprised of a positive and negative electrode which are both in contact with an aqueous metal-halide electrolyte. The electrolyte circulation subsystem operates to circulate the metal-halide electrolyte from a reservoir through each of the cells in the electrode stack in order to replenish the metal and halogen electrolyte ionic components as they are oxidized or reduced in the cells during the battery cycle. In a closed, self-contained metal-halogen battery system, the storage subsystem is used to contain the halogen gas or liquid which is liberated from the cells during the charging of the battery system for subsequent return to the cells during the discharging of the battery system. In the zinc-chloride battery system, chlorine gas is liberated from the positive electrodes of the cells and stored in the form of chlorine hydrate. Chlorine hydrate is a solid which is formed by the store subsystem in a process analogous to the process of freezing water where chlorine is inclined in the ice crystal.
With reference to the general operation of a zinc-chloride battery system, an electrolyte pump operates to circulate the aqueous zinc-chloride electrolyte from a reservoir to each of the positive or "chlorine" electrodes in the electrode stack. These chlorine electrodes are typically made of porous graphite, and the electrolyte passes through the pores of the chlorine electrodes into a space between the chlorine electrodes and the opposing negative or "zinc" electrodes. The electrolyte then flows up between the opposing electrodes or otherwise out of the cells in the electrode stack and back to the electrolyte reservoir or sump.
During the charging of the zinc-chloride battery system, zinc metal is deposited on the zinc electrode substrates and chlorine gas is liberated or generated at the chlorine electrode. The chlorine gas is collected in a suitable conduit, and then mixed with a chilled liquid to form chlorine hydrate. A gas pump is typically employed to draw the chlorine gas from the electrode stack and mix it with the chilled liquid, (i.e., generally either zinc-chloride electrolyte or water). The chlorine hydrate is then deposited in a store container until the battery system is to be discharged.
During the discharging of the zinc-chloride battery system, the chlorine hydrate is decomposed by permitting the store temperature to increase, such as by circulating a warm liquid through the store container. The chlorine gas thereby recovered is returned to the electrode stack via the electrolyte circulation subsystem, were it is reduced at the chlorine electrodes. Simultaneously, the zinc metal is dissolved off of the zinc electrode substrates, and power is available at the battery terminals.
Over the course of the zinc-chloride battery charge/discharge cycle, the concentration of the electrolyte varies as a result of the electrochemical reactions occurring at the electrodes in the cells of the electrode stack. At the beginning of charge, the concentration of zinc-chloride in the aqueous electrolyte may typically be 2.0 molar. As the charging portion of the cycle progresses, the electrolyte concentration will gradually decrease with the depletion of zinc and chloride ions from the electrolyte. When the battery system is fully charged, the electrolyte concentration will typically be reduced to 0.5 molar. Then, as the battery system is discharged, the electrolyte concentration will gradually swing upwardly and return to the original 2.0 molar concentration when the battery system is completely or fully discharged.
Further discussion of the structure and operation of zinc-chloride battery systems may be found in the following commonly assigned patents: Symons U.S. Pat. No. 3,713,888 entitled "Process For Electrical Energy Using Solid Halogen Hydrates"; Symons U.S. Pat. No. 3,809,578 entitled "Process For Forming And Storing Halogen Hydrate In A Battery"; Carr et al U.S. Pat. No. 3,909,298 entitled "Batteries Comprising Vented Electrodes And Method of Using Same"; Carr U.S. Pat. No. 4,100,332 entitled "Comb Type Bipolar Electrode Elements And Battery Stack Thereof". Such systems are also described in published reports prepared by the assignee herein, such as "Development of the Zinc-Chloride Battery for Utility Applications", Interim Report EM-1417, May 1980, and "Development of the Zinc-Chloride Battery for Utility Applications", Interim Report EM-1051, April 1979, both prepared for the Electric Power Research Institute, Palo Alto, Calif. The specific teachings of the aforementioned cited references are incorporated herein by reference.
The present invention is directed to an improved comb-type bipolar cell and battery stack design. Specifically, it is a principal objective of the present invention to provide an improved cell design which is based upon flat plate electrodes (i.e., electrodes not having a contoured faces).
It is another objective of the present invention to provide an improved cell design which incorporates low-cost electrode frames that may be injection molded to enhance manufacturability and which facilitates assembly of the battery stack.
It is a further objective of the present invention to provide an improved cell design which incorporates low-cost electrode frames that feature integral masking of both the interior and exterior of the positive and negative electrodes.
It is an additional objective of the present invention to provide an improved cell design which incorporates low-cost electrode frames which features an electrolyte feed tube providing a controlled flow of electrolyte to the positive electrodes.
It is a further objective of the present invention to provide improved unit cell and battery stack design to provide a controlled flow of electrolyte that is uniform to all positive electrodes within the battery stack.
It is an objective of the present invention to provide an improved electrode frame design which physically supports electrodes placed therein so as to prevent electrode bowing and to maintain the desired inter-electrode gap.
It is another objective of the present invention to provide an improved unit cell and battery stack design having excellent hydraulic integrity and ionic cell-to-cell isolation.
It is yet another objective of the present invention to provide an improved cell design which is capable of filtering the electrolyte flow to a plurality of cells connected electrically in parallel.
It is still another objective of the present invention to provide a cell design which incorporates a substantially closed compartment which controls both the electrolyte flow to and from a plurality of cells.
The present invention also incorporates the above electrode assembly into a comb-type bipolar cell design which ionically isolates adjacent cells which are connected electrically in series. Such ionic isolation is important in achieving uniform coulombic efficiency for each of the cells connected electrically in series. This cell design also features a housing or compartment means having a top section which includes manifold means for controlling both the electrolyte flow into and out of each series connected cell.
Additional advantages and features for the present invention will become apparent from a reading of the detailed description of the preferred embodiments which make reference to the following set of drawings in which :